First seen as a lovely dressed young woman who looks a lot like Chizuru.

Spoiler

Kaoru is actually Chizuru's twin brother who was separated at birth. He was dressed as a woman. He wants Chizuru to suffer, since no one ever protected him.

Spoiler

Later on, though, he admits that he truly loves Chizuru and that everything he had done was to meet Chizuru again. In addition, he asks Chizuru to go with him, because he is the only one who has the power to protect her. Chizuru refuses to go, since she had promised to the Shinsengumi that she would protect Kondo, and Kaoru decides that if things had come to that, his only option would be killing her. Right after they start fighting, though, Chizuru is cornered and saved by Chikage, who kills Kaoru.

Source: Wikipedia